Directivity at Optical Frequencies Using Nanoantenna Array

Abstract

Directivity has played a vital role in telecom industry by enabling directional radiation of electromagnetic energy thus making satellite communication a possibility. The theory of directional radiation has been rigorously investigated at radio frequencies (RF) and microwave and design guidelines are well established. Here we show a method for the translation of design rules from microwave theory to optical antenna design. At optical frequencies, metal can no longer be treated as a perfect conductor which makes the design translation nontrivial. Plasmonic effects come into play requiring design modifications which are not predicted by RF/microwave theory. In this work, we present the design translation of antenna arrays from microwave to optical regime by considering plasmonic effects as well as material losses using dipole antennas as a model system. Finite difference time domain numerical simulations are carried out to study the array of optical dipole antenna design. We first show that a single dipole antenna is significantly shorter than half a wavelength at optical frequencies in contrast to microwave theory. We further report beam forming using array of optical dipole antennas. Translation of proposed design guidelines and techniques to optical regime could lead to improved device performance in the areas of solar energy conversion, spectroscopy and nanophotonics.